Sheet Metal Tolerance

When working with clients with these types of projects (tight tolerances referencing less then ideal datums) I always create two go/no go guides. (I add them into my initial quote and explain why they are important for both myself and the client) I keep one for myself and give one to the client for their QC department. IMO this is money well spent. The gauges help eliminate arguments over if parts are made to print. Otherwise with flexible parts like you said it's too easy to add deflection/distortion to the parts changing measurements.

I think before going in QC inspection of production it is very important that you develop right and good tool to get the defect free products.

I strongly belive that all the quality problems those we face in every day are due to poor process and design of components.

In 50% of cases components are rejected yet they dont deserve to be rejected.

this happen because somebody doesnt take care of defining the right specs and tolerancing.

rest of probelms are attributed to the desing development and tooling department.

Very rearly it happen that an ordinary worker creat good or bad quality.

So advice, go for kill at tooling and development level.

I think you should be bit precise when you provide some info.

what is the unit?

I Guess it in inches.

also what is the size of holes because tolararce have some sort of relationship with the dimensions.

Also I am not clear why you want to do machining?

where as sheet metal can be punched on press tool with a good accuracy.

you have not indicated that are these tolerance on the CDs of holes or on hole dimensions itself.

If you talk about .004 Inches and 0.009 inches these are not too tight for a press tooled component.

Most important is it functionally correct, does the mating part fit in its proper location, +/- .009 is a normal tolerlance. often, I've seen inspection using a incorrect method of checking a part. Clamped down or in a free state? Try to work with the customer, some parts are just over toleranced, maybe geometric tolerance is the way to go.

Jim C

The "go/no go" is the best way. Get with the customer and find out which way all of the torences must be perfect and measured exactly from where and at what temperature. Then jig your tooling the same way. There is no reason to miss by more than .006".

What is the material? steel or aluminum? You also mentioned welding, warpage due to heat is hard to acount for. Form fit and function may need to be the standard .

Thank you for all your comments. Let me answer first answer your questions.

The box is made from 5052 aluminum. The tolerances are in inches and they are for the location of the holes, not the hole sizes.

The sheet metal shops usually make all the holes in the flat which their press machines can typically hold to +/-.005". They then bend and weld up the box. The distance between holes on any side of the box should therefore meet the +/-.005" tolerance.

Unfortunately, the tolerance from a bent surface to a hole can typically only be held to +/-.01" (most places prefer +/-.02"). My hole locations are measured from a bent surface. The holes therefore had to be machined after the box had been bent and welded to hold the tighter tolerances.

I have used geometric tolerancing and have worked with the vendors and QA to specify exactly how the holes should be measured. The persons machining the holes are aware of how they will be measured at inspection and adjusted for this as much as possible.

The go-no go guide seems like the best approach for future projects but I will make every effort in the future to avoid machining tightly located holes in sheet metal. Instead, I will use large, loosely located holes in the sheet metal and use locational jigs to locate the parts that attach to these holes to meet the end use requirements. This is what I have typically done in the past with good success.

Can you convince you and your customer to agree on the conditions of fixity? If the part is that flimsy anyway it should matter in thye higher assembly. You may just be chasing "ghost" dimensions.